In the last few decades, progress in radio and Very Large Scale Integration (VLSI) technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as mobile radios, can now be produced having acceptable cost, size and power consumption. Mobile phone communications for the consumer market started with phone systems derived from the police and rescue services and was based on an analog technology improved and optimized in the seventies and eighties. Examples of these analog phone systems are Nordic Mobile Telephone (NMT) and Total Access Communications System (TACS). The usage of mobile phones really took off in the nineties with the introduction of mobile phone systems based on digital technology like Global System for Mobile Communications (GSM), Digital Advanced Mobile Phone Service (D-AMPS) and Personal Digital Cellular (PDC). Generally, the analog systems are indicated as being the first generation, whereas the digital systems are indicated as second generation.
Currently, third generation systems are being developed. Two different development paths can be distinguished. One path considers completely new systems based on a technology that differs from the previous generation. For example, the third generation system, Universal Mobile Telecommunication System (UMTS) being developed in Europe, Asia and the US, though digital, is based on wideband transmission whereas most second generation systems are based on narrowband transmission. The other path considers high data rate (used interchangeably with high-rate(s) hereinafter) in the existing second-generation systems. An example is the EDGE mode applied in GSM and in D-AMPS. In the latter case, a second-generation system is updated to a third generation system through the use of high rate modes.
This general trend of updating an existing system by adding dedicated functionality like higher data rates is attractive since the system basic functions like control and mobility support can still be based on the conventional system operation whereas only dedicated modes make use of the advanced features. Compatibility with existing portable devices is guaranteed.
A limitation in the current solution is that, in order to establish a dedicated high-speed link, both sender and receiver must be capable of supporting the higher data rates. Although trivial at first sight, this is not at all the case when it is understood that for many of the applications where high data rates are desired, the increase in data rate is desired in one direction only. For example when a laptop downloads a file to a printer via a short range radio link like Bluetooth, the increased data rate is required only in the direction from the laptop to the printer. The return channel from the printer to the laptop is only required to support basic data link control (DLC) messages and other link related signaling. The return channel can easily be supported by the conventional radio link. It will be understood that an information sink like a printer preferably has an advanced receiver for high-rate reception. The transmitter, though, can be of a simple type as only low data rates (used interchangeably with low-rate(s) hereinafter) have to be supported in the transmit direction seen from the printer. Alternatively, examples can be found of typical information sources.
Consider a digital camera. The user wants to transfer the pictures out of the camera at high speed but will never transfer pictures into the camera. Therefore, the camera preferably has an advanced transmitter for supporting high data rates whereas the receiver can be fairly simple. Since the complexity of a high-rate radio transmitter is very different from the complexity of a high-rate radio receiver, it is recognized that the benefit of adding high-speed modes can be improved by taking into account the asymmetric speed requirements.
Therefore, there is a need in the art for a system where high-speed modes can be supported with reduced costs if asymmetry in the speed requirements is present.